It's indeed possible to solder karat gold findings to the karat
gold finish on gold filled sheet, etc. I would suggest the
thicker gauges of gold filled because, remember, the karat gold
only amounts to 5% of the material. Therefore, the thicker the
gauge, the thicker the gold layer. This is of primary concern
when soldering when you consider the potential for melting that
thin layer if too much heat is applied. Also, a clean solder job
is necessary to avoid much abrasive action required on the
surface for clean up, remember, that layer is mighty thin.
However, I've soldered numerous 14K findings onto gold filled and
would suggest you give it a try.